Measuring RF Noise Immunity Using an Anechoic Chamber

The widespread popularity of GSM mobile phones has led to a continuous increase in unwanted RF signals. If the electronic circuit does not have sufficient RF suppression capability, these RF signals will cause the circuit to distort the results. In order to ensure the reliable operation of electronic circuits, the measurement of the RF suppression capability of electronic circuits has become an indispensable part of product design. This article introduces a universal RF suppression capability measurement technology - RF anechoic chamber measurement device, describes its composition and operation, and gives examples of actual measurement results.

The wireless technology used in most cell phones today is time division multiple access (TDMA), a multiplexing technique that modulates a high frequency carrier wave with on/off pulses at a frequency of 217 Hz. ICs that are susceptible to RF interference will demodulate this carrier signal and regenerate signals at 217 Hz and its harmonics. Since most of these spectral components fall into the audio range, they produce an unwanted audible "buzz." As a result, circuits with poor RF immunity will demodulate the RF signal from the cell phone and produce undesirable low-frequency noise. In order to measure the quality of the product, the circuit needs to be tested in an RF environment that is equivalent to the environment encountered by the circuit during normal operation.

This article describes a general technique for measuring the RF noise immunity of integrated circuits. RF immunity testing subjects the board to controlled RF signal levels that represent the level of interference that the circuit may experience during operation. This results in a standardized, structured test method that can produce useful results that are repeatable in quality analysis. Such test results can help select ICs to obtain the circuits that are most immune to RF noise.

The device under test (DUT) can be brought close to a working cell phone to test its RF sensitivity, but in order to obtain an accurate and repeatable test result, a fixed measurement method is required to test the DUT in a repeatable RF field. The solution is to use an RF test anechoic chamber, which provides a precisely controlled RF field, which is equivalent to the RF field generated by a typical mobile phone.

Below, we compare the RF immunity of a Maxim dual op amp (MAX4232) and a competitive product (X).

Figure 1: Dual op amp RF noise suppression measurement circuit (online)

The RF rejection test circuit (Figure 1) shows the board connections to the dual op amps under test. Each op amp is configured as an AC amplifier with the output set to a 1.5V DC level (VCC = 3V) when no AC input is present. The inverting input is shorted to ground via a 1.5" loop (PC leads at the analog inputs). This loop is used to simulate the effect of real leads, which would act as an antenna at the operating frequency to collect and demodulate RF signals. By connecting a voltmeter to the output, the RF noise rejection capability of the op amp is measured and quantified.

Figure 2: RF noise immunity measurement setup

Maxim's RF test setup (Figure 2) generates the RF field required for RF immunity testing. The test anechoic chamber has a shielded room that acts like a Faraday cavity, and it has ports for connecting power supplies and output monitors. The test setup is constructed by connecting the following equipment:

1. Signal generator: 9kHz to 3.3GHz (Rohde & Schwarz SML-03)
2. RF power amplifier (PA): 800MHz to 1GHz, 20W (OPHIR 5124)
3. Power meter: 25MHz to 1GHz (Rohde & Schwarz)
4. Parallel line unit (anechoic chamber)
5. Field strength detector
6. Computer
7. Voltmeter

The signal generator generates an RF modulated signal of the required frequency and feeds it to the power amplifier. The PA output is measured and monitored through a directional coupler connected to a power meter. The computer establishes the required RF field by controlling the frequency range, modulation type, modulation percentage, and power output of the signal generator output. The electric field is radiated in a shielded anechoic chamber through an antenna (planar type) and is precisely calibrated to produce a uniform, consistent, and repeatable electric field.

The RF field strength near a typical cell phone is approximately 60V/m (4cm from the phone antenna), and the field strength decreases as you move away from the phone. At 10cm from the phone, the field strength drops to 25V/m. Therefore, a uniform 60V/m field strength is generated in the anechoic chamber to simulate the RF environment in which the DUT is located (the 60V/m radiation intensity ensures that the device under test will not be level clamped, avoiding measurement errors). The RF signal used is an RF sine wave that varies in the 800MHz to 1GHz cell phone frequency range, modulated with an audio frequency of 1kHz and a modulation depth of 100%. Similar results can be obtained when modulating with a frequency of 217Hz, but 1kHz is a more common audio frequency and was chosen here for ease of estimation. Power is provided to the DUT through the access port of the anechoic chamber, and a voltmeter is connected through the access port to read the dBV value (dB relative to 1V). The RF field can be accurately calibrated by adjusting the position of the DUT in the anechoic chamber and using a field strength tester.

Figure 3. RF noise rejection test results of two dual op amps using the measurement setup of Figure 2.

The test results of the two dual op amps (MAX4232 and X) are shown in Figure 3, and the measured values ​​are the average dBV of the output. When the RF frequency varies from 800MHz to 1GHz, in a uniform 60V/m electric field, the average output of the MAX4232 is about -66dBV (500(V RMS relative to 1V); the average output of device X is about (18dBV (125mV RMS relative to 1V). When there is no RF signal, the voltmeter reads -86dBV.

Therefore, the output of the MAX4232 changes by only -20dB (-86dBV to (66dBV), which means that the RF interference causes the MAX4232 output to change from 50(V RMS) to 500(V RMS). In the RF interference environment, the MAX4232 changes by a factor of 10. Therefore, it can be inferred that the MAX4232 has excellent RF immunity (-66dBV) and does not produce significant output distortion. The average noise immunity reading of device X is only -18dBV, which means that the output changes by 125mV RMS (relative to 1V) under the influence of RF. This increase is a large value, 2500 times the normal value of 50(V RMS). Therefore, device X has poor RF noise immunity (-18dBV) and may not work properly when close to mobile phones or other RF sources. Obviously, for audio processing applications (such as headphone amplifiers and microphone amplifiers), the MAX4232 is a better choice.

In order to ensure the working quality of the product in the RF environment, the measurement of RF noise suppression capability is a step that circuit board and IC manufacturers must consider. RF anechoic chamber measurement equipment provides an economical, flexible and accurate RF suppression capability measurement method.